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Discrete Convolution


I want a function that mimics Matlab's "conv" function for doing a discrete
convolution of two lists.

 CONV Convolution and polynomial multiplication.
    C = CONV(A, B) convolves vectors A and B.  The resulting
    vector is length LENGTH(A)+LENGTH(B)-1.
    If A and B are vectors of polynomial coefficients, convolving
    them is equivalent to multiplying the two polynomials.


I wrote the following, but is there a way of either of
(1) speeding up the code by changing the algorithm ...
      ignoring simple things like the multiple evaluations
      of Length and so forth which I have left
      in only for what I hope is clarity; or
(2) Using a built in function (possibly connected with polynomials) to do
the same thing?

Mark R Diamond
No spam email: markd at psy dot uwa dot edu dot au
--------------------------------------------------------

convolve[a_List,b_List]:=Module[
  {
       (* reverse one of the lists prior to the convolution *)
       ra=Reverse[a],

       (* A variable that collects the indices of lists ra and b,
respectively *)
       (* that will be Dot[ ]-ed together. *)
       indices
  },

  (* Create the table of indices *)
  indices=Table[
   {
    {
      Max[Length[a]+1-i,1],
      Min[Length[a],Length[a]+Length[b]-i]
    },
     {
      Max[1,i-Length[a]+1],Min[Length[b],i]
     }
   },
   {i,Length[a]+Length[b]-1}
  ];

  (* Create a list of the appropriate pairs of dot products *)
   Map[(Take[ra,#[[1]] ].Take[ b,#[[2]] ])&, indices]
 ]  /;  (VectorQ[a,NumberQ]\[And]VectorQ[b,NumberQ])




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